Core configuration for in-situ electromagnetic induction monitoring system

ABSTRACT

An apparatus for chemical mechanical polishing includes a support for a polishing pad having a polishing surface, and an electromagnetic induction monitoring system to generate a magnetic field to monitor a substrate being polished by the polishing pad. The electromagnetic induction monitoring system includes a core and a coil wound around a portion of the core. The core includes a back portion, a center post extending from the back portion in a first direction normal to the polishing surface, and an annular rim extending from the back portion in parallel with the center post and surrounding and spaced apart from the center post by a gap. A width of the gap is less than a width of the center post, and a surface area of a top surface of the annular rim is at least two times greater than a surface area of a top surface of the center post.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 62/411,407, filed on Oct. 21, 2016, and claims priority to U.S.Provisional Application Ser. No. 62/415,641, filed on Nov. 1, 2016, eachof which is incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to electromagnetic induction monitoring,e.g., eddy current monitoring, during processing of a substrate.

BACKGROUND

An integrated circuit is typically formed on a substrate (e.g. asemiconductor wafer) by the sequential deposition of conductive,semiconductive or insulative layers on a silicon wafer, and by thesubsequent processing of the layers.

One fabrication step involves depositing a filler layer over anon-planar surface, and planarizing the filler layer until thenon-planar surface is exposed. For example, a conductive filler layercan be deposited on a patterned insulative layer to fill the trenches orholes in the insulative layer. The filler layer is then polished untilthe raised pattern of the insulative layer is exposed. Afterplanarization, the portions of the conductive layer remaining betweenthe raised pattern of the insulative layer form vias, plugs and linesthat provide conductive paths between thin film circuits on thesubstrate. In addition, planarization may be used to planarize thesubstrate surface for lithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier head. The exposed surface of thesubstrate is placed against a rotating polishing pad. The carrier headprovides a controllable load on the substrate to push it against thepolishing pad. A polishing liquid, such as slurry with abrasiveparticles, is supplied to the surface of the polishing pad.

During semiconductor processing, it may be important to determine one ormore characteristics of the substrate or layers on the substrate. Forexample, it may be important to know the thickness of a conductive layerduring a CMP process, so that the process may be terminated at thecorrect time. A number of methods may be used to determine substratecharacteristics. For example, optical sensors may be used for in-situmonitoring of a substrate during chemical mechanical polishing.Alternately (or in addition), an eddy current sensing system may be usedto induce eddy currents in a conductive region on the substrate todetermine parameters such as the local thickness of the conductiveregion.

SUMMARY

In one aspect, an apparatus for chemical mechanical polishing includes asupport for a polishing pad having a polishing surface, and anelectromagnetic induction monitoring system to generate a magnetic fieldto monitor a substrate being polished by the polishing pad. Theelectromagnetic induction monitoring system includes a core and a coilwound around a portion of the core. The core includes a back portion, acenter post extending from the back portion in a first direction normalto the polishing surface, and an annular rim extending from the backportion in parallel with the center post and surrounding and spacedapart from the center post by a gap. The center post has a first widthin a second direction parallel to the polishing surface, the annular rimhas a second width in the second direction and the gap has a third widthin the second direction. The third width is less than the first width,and a surface area of a top surface of the annular rim is at least twotimes greater than a surface area of a top surface of the center post.

Implementations may include one or more of the following features.

The second width may be greater than the first width. The second widthmay be 1.1 to 1.5 times greater than the first width. The third widthmay be 50% to 75% of the first width. The surface area of the topsurface of the annular rim may be at least three times greater than thesurface area of the top surface of the center post. A height of thecenter post may be equal to a height of the annular rim portion. Thethird width may be between about 30% and 70% of the second width. Thecoil and core may be configured to provide a resonant frequency of atleast 12 MHz, e.g., between about 14 and 16 MHz. The core may be nickelzinc ferrite.

In another aspect, an apparatus for chemical mechanical polishingincludes a support for a polishing pad having a polishing surface, andan electromagnetic induction monitoring system to generate a magneticfield to monitor a substrate being polished by the polishing pad. Theelectromagnetic induction monitoring system includes a core and awinding assembly. The core includes a back portion, a center postextending from the back portion in a first direction normal to thesurface of the platen, and an annular rim extending from the backportion in parallel with the center post and surrounding and spacedapart from the center post by a gap. The center post has a first widthin a second direction parallel to the surface of the platen, the annularrim has a second width in the second direction, and the gap has a thirdwidth in the second direction. The winding assembly is a cylindricalbody fitting in the gap. The winding assembly includes a coil woundaround the center post, and the winding assembly has a fourth widthbetween an inner diameter and an outer diameter of the cylindrical body.The fourth width is at least 80% of the third width.

Implementations may include one or more of the following features.

The winding assembly may include a bobbin, the coil may be wound aroundthe bobbin, and an inner surface of the bobbin may provide the innerdiameter of the winding assembly. The inner surface of the bobbin maycontact an outer surface of the center post. The winding assembly mayinclude a tape contacting and surrounding the coil, and an outer surfaceof the tape may provide the outer diameter of the winding assembly. Theouter surface of the tape may contact an inner surface of the annularrim.

The coil may have no more than two winding layers around the centerpost, e.g., the coil may have a single winding layer around the centerpost. The fourth width may be at least 90% of the third width. The thirdwidth may be about 1 to 2 mm. The third width may be less than the firstwidth, and a surface area of a top surface of the annular rim may be atleast two times greater than a surface area of a top surface of thecenter post.

Certain implementations can include one or more of the followingadvantages. Spatial resolution of the eddy current sensor can beimproved. The eddy current sensor can be configured for monitoring ofconductive features that have a high impedance, e.g., metal sheetsformed of a low conductance metal such as titanium or cobalt, metalresidue, or metal lines.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other aspects, featuresand advantages will be apparent from the description and drawings, andfrom the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side view, partially cross-sectional, of achemical mechanical polishing station that includes an electromagneticinduction monitoring system.

FIG. 2 is a schematic top view of the chemical mechanical polishingstation of FIG. 1.

FIGS. 3A-3C are schematic cross-sectional side views illustrating amethod of polishing a substrate.

FIG. 4 is a schematic circuit diagram of a drive system for anelectromagnetic induction monitoring system.

FIGS. 5A and 5B are schematic top and side views of the core of theelectromagnetic induction monitoring system.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

A CMP system can use an eddy current monitoring systems to detect athickness of a metal layer on a substrate during polishing. Duringpolishing of the metal layer, the eddy current monitoring system candetermine the thickness of the metal layer in different regions of thesubstrate. The thickness measurements can be used to detect thepolishing endpoint or to adjust processing parameters of the polishingprocess in real time to reduce polishing non-uniformity.

One issue with eddy current monitoring is that the eddy current isinduced in the conductive layer in a region whose size depends on thespread of the magnetic field; the greater the spread of the magneticfield, the lower the resolution of the eddy current monitoring system.With ever increasing demands of integrated circuit fabrication, there isa need for increased spatial resolution of the eddy current sensor,e.g., in order to provide improved control of the polishing parameters.Appropriate selection of the physical configuration of the magnetic corecan reduce the spread of the magnetic field and provide improvedresolution.

FIGS. 1 and 2 illustrate an example of a polishing station 20 of achemical mechanical polishing apparatus. The polishing station 20includes a rotatable disk-shaped platen 24 on which a polishing pad 30is situated. The platen 24 is operable to rotate about an axis 25. Forexample, a motor 22 can turn a drive shaft 28 to rotate the platen 24.The polishing pad 30 can be a two-layer polishing pad with an outerpolishing layer 34 and a softer backing layer 32.

The polishing station 22 can include a supply port or a combinedsupply-rinse arm 39 to dispense a polishing liquid 38, such as slurry,onto the polishing pad 30. The polishing station 22 can include a padconditioner apparatus with a conditioning disk to maintain the surfaceroughness of the polishing pad.

The carrier head 70 is operable to hold a substrate 10 against thepolishing pad 30. The carrier head 70 is suspended from a supportstructure 72, e.g., a carousel or a track, and is connected by a driveshaft 74 to a carrier head rotation motor 76 so that the carrier headcan rotate about an axis 71. Optionally, the carrier head 70 canoscillate laterally, e.g., on sliders on the carousel or track 72; or byrotational oscillation of the carousel itself.

In operation, the platen is rotated about its central axis 25, and thecarrier head is rotated about its central axis 71 and translatedlaterally across the top surface of the polishing pad 30. Where thereare multiple carrier heads, each carrier head 70 can have independentcontrol of its polishing parameters, for example each carrier head canindependently control the pressure applied to each respective substrate.

The carrier head 70 can include a flexible membrane 80 having asubstrate mounting surface to contact the back side of the substrate 10,and a plurality of pressurizable chambers 82 to apply differentpressures to different zones, e.g., different radial zones, on thesubstrate 10. The carrier head can also include a retaining ring 84 tohold the substrate.

A recess 26 is formed in the platen 24, and optionally a thin section 36can be formed in the polishing pad 30 overlying the recess 26. Therecess 26 and thin pad section 36 can be positioned such that regardlessof the translational position of the carrier head they pass beneathsubstrate 10 during a portion of the platen rotation. Assuming that thepolishing pad 30 is a two-layer pad, the thin pad section 36 can beconstructed by removing a portion of the backing layer 32, andoptionally by forming a recess in the bottom of the polishing layer 34.The thin section can optionally be optically transmissive, e.g., if anin-situ optical monitoring system is integrated into the platen 24.

Referring to FIG. 3A, the polishing system 20 can be used to polish asubstrate 10 that includes a conductive material overlying and/or inlaidin a patterned dielectric layer. For example, the substrate 10 caninclude a layer of conductive material 16, e.g., a metal, e.g., copper,aluminum, cobalt or titanium, that overlies and fills trenches in adielectric layer 14, e.g., silicon oxide or a high-k dielectric.Optionally a barrier layer 18, e.g., tantalum or tantalum nitride, canline the trenches and separate the conductive material 16 from thedielectric layer 14. The conductive material 16 in the trenches canprovide vias, pads and/or interconnects in a completed integratedcircuit. Although the dielectric layer 14 is illustrated as depositeddirectly on a semiconductor wafer 12, one or more other layers can beinterposed between the dielectric layer 14 and the wafer 12.

Initially, the conductive material 16 overlies the entire dielectriclayer 14. As polishing progresses, the bulk of the conductive material16 is removed, exposing the barrier layer 18 (see FIG. 3B). Continuedpolishing then exposes the patterned top surface of the dielectric layer14 (see FIG. 3C). Additional polishing can then be used to control thedepth of the trenches that contain the conductive material 16.

Returning to FIG. 1, the polishing system 20 includes an in-situelectromagnetic induction monitoring system 100 which can be coupled toor be considered to include a controller 90. A rotary coupler 29 can beused to electrically connect components in the rotatable platen 24,e.g., the sensors of the in-situ monitoring systems, to componentsoutside the platen, e.g., drive and sense circuitry or the controller90.

The in-situ electromagnetic induction monitoring system 100 isconfigured to generate a signal that depends on a depth of theconductive material 16, e.g., the metal. The electromagnetic inductionmonitoring system can operate either by generation of eddy-currents inthe conductive material, which can be either the sheet of conductivematerial that overlies the dielectric layer or the conductive materialremaining in trenches after the dielectric layer is exposed, orgeneration of current in a conductive loop formed in a trench in thedielectric layer on the substrate.

In operation, the polishing system 20 can use the in-situ monitoringsystem 100 to determine when the conductive layer has reached a targetthickness, e.g., a target depth for metal in a trench or a targetthickness for a metal layer overlying the dielectric layer, and thenhalts polishing. Alternatively or in addition, the polishing system 20can use the in-situ monitoring system 100 to determine differences inthickness of the conductive material 16 across the substrate 10, anduses this information to adjust the pressure in one or more chambers 82in the carrier head 80 during polishing in order to reduce polishingnon-uniformity.

The in-situ monitoring system 100 can include a sensor 102 installed ina recess 26 in the platen 24. The sensor 102 can include a magnetic core104 positioned at least partially in the recess 26, and at least onecoil 106 wound around a portion of the core 104. Drive and sensecircuitry 108 is electrically connected to the coil 106. The drive andsense circuitry 108 generates a signal that can be sent to thecontroller 90. Although illustrated as outside the platen 24, some orall of the drive and sense circuitry 108 can be installed in the platen24.

Referring to FIG. 2, as the platen 24 rotates, the sensor 102 sweepsbelow the substrate 10. By sampling the signal from the circuitry 108 ata particular frequency, the circuitry 108 generates measurements at asequence of sampling zones 94 across the substrate 10. For each sweep,measurements at one or more of the sampling zones 94 can be selected orcombined. Thus, over multiple sweeps, the selected or combinedmeasurements provide the time-varying sequence of values.

The polishing station 20 can also include a position sensor 96, such asan optical interrupter, to sense when the sensor 102 is underneath thesubstrate 10 and when the sensor 102 is off the substrate. For example,the position sensor 96 can be mounted at a fixed location opposite thecarrier head 70. A flag 98 can be attached to the periphery of theplaten 24. The point of attachment and length of the flag 98 is selectedso that it can signal the position sensor 96 when the sensor 102 sweepsunderneath the substrate 10.

Alternately or in addition, the polishing station 20 can include anencoder to determine the angular position of the platen 24.

Returning to FIG. 1, a controller 90, e.g., a general purposeprogrammable digital computer, receives the signals from the in-situmonitoring system 100. Since the sensor 102 sweeps beneath the substrate10 with each rotation of the platen 24, information on the depth of theconductive layer, e.g., the bulk layer or conductive material in thetrenches, is accumulated in-situ (once per platen rotation). Thecontroller 90 can be programmed to sample measurements from the in-situmonitoring system 100 when the substrate 10 generally overlies thesensor 102.

In addition, the controller 90 can be programmed to calculate the radialposition of each measurement, and to sort the measurements into radialranges.

FIG. 4 illustrates an example of the drive and sense circuitry 108. Thecircuitry 108 applies an AC current to the coil 106, which generates amagnetic field 150 between two poles 152 a and 152 b of the core 104. Inoperation, when the substrate 10 intermittently overlies the sensor 104,a portion of the magnetic field 150 extends into the substrate 10.

The circuitry 108 can include a capacitor 160 connected in parallel withthe coil 106. Together the coil 106 and the capacitor 160 can form an LCresonant tank. In operation, a current generator 162 (e.g., a currentgenerator based on a marginal oscillator circuit) drives the system atthe resonant frequency of the LC tank circuit formed by the coil 106(with inductance L) and the capacitor 160 (with capacitance C). Thecurrent generator 162 can be designed to maintain the peak to peakamplitude of the sinusoidal oscillation at a constant value. Atime-dependent voltage with amplitude V0 is rectified using a rectifier164 and provided to a feedback circuit 166. The feedback circuit 166determines a drive current for current generator 162 to keep theamplitude of the voltage V0 constant. Marginal oscillator circuits andfeedback circuits are further described in U.S. Pat. Nos. 4,000,458, and7,112,960.

As an eddy current monitoring system, the electromagnetic inductionmonitoring system 100 can be used to monitor the thickness of aconductive layer by inducing eddy currents in the conductive sheet, orto monitor the depth of a conductive material in a trench by inducingeddy currents in the conductive material. Alternatively, as an inductivemonitoring system, the electromagnetic induction monitoring system canoperate by inductively generating a current in a conductive loop formedin the dielectric layer 14 of the substrate 10 for the purpose ofmonitoring, e.g., as described in U.S. Patent Publication No.2015-0371907.

If monitoring of the thickness of a conductive layer on the substrate isdesired, then when the magnetic field 150 reaches the conductive layer,the magnetic field 150 can pass through and generate a current (if thetarget is a loop) or create an eddy-current (if the target is a sheet).This creates an effective impedance, thus increasing the drive currentrequired for the current generator 162 to keep the amplitude of thevoltage V0 constant. The magnitude of the effective impedance depends onthe thickness of the conductive layer. Thus, the drive current generatedby the current generator 162 provides a measurement of the thickness ofthe conductive layer being polished.

Other configurations are possible for the drive and sense circuitry 108.For example, separate drive and sense coils could be wound around thecore, the drive coil could be driven at a constant frequency, and theamplitude or phase (relative to the driving oscillator) of the currentfrom the sense coil could be used for the signal.

FIGS. 4A and 4B illustrate an example of a core 104 for the in-situmonitoring system 100. The core 104 has a body formed of anon-conductive material with a relatively high magnetic permeability(e.g., μ of about 2500 or more). Specifically, the core 104 can benickel-zinc ferrite or magnesium-zinc ferrite.

In some implementations, the core 104 is coated with a protective layer.For example, the core 104 can be coated with a material such as paryleneto prevent water from entering pores in the core 104, and to preventcoil shorting.

The core 104 can be round core, also known as a pot core. The core 104includes a back portion 120, a center post 122 that extends from theback portion 120, and an annular rim 124 surrounding and spaced apartfrom the center post 122 by a gap 126 and also extending from the backportion 120. The annular rim 124 can be spaced apart from the centerpost 122 by a uniform distance around the perimeter of the center post122. The annular rim 124 can completely enclose the center post 122 (asseen in the top view of FIG. 4B).

A winding assembly 130 fits into the gap 126. The winding assembly can acylindrical body. The winding assembly has a width (W4), which can bethe distance between an inner diameter and an outer diameter of thecylindrical body.

The winding assembly 130 includes at least the coil 106, which is woundaround the center post 122 of the core 104, e.g., only around the centerpost 122. In order to reduce the required width of the gap 126, the coil106 can have just one or two layers of windings.

The winding assembly 130 can also include a bobbin 132. The bobbin 132fits around the center post 122, and the coil 106 is wound around thebobbin 132. The bobbin 132 can also include a cap 136 that rests againstthe top surface of the post 124 to set the vertical position of the coilportion. This permits easier assembly of the sensor 102. The bobbin canbe a dielectric material, e.g., a plastic. The inner surface of thebobbin 132 can provide the outer diameter of the winding assembly.

The winding assembly 130 can also include a tape 134 that covers theouter surface of the coil 106, e.g., to protect the coil 106. The outersurface of the tape 134 can provide the inner diameter of the windingassembly.

The back portion 120 of the core 106 can be a generally planar body andcan have a top face parallel to the top surface of the platen, e.g.,parallel to the substrate and the polishing pad during the polishingoperation. The back portion 120 can have a height (H) that is measurednormal to the top surface of the platen. The center post 122 and theannular rim 124 extend from the back portion 120 in a direction normalto a top surface of the back portion 120 and extend in parallel witheach other. The center post 122 and the annular rim 124 can have thesame height.

In some implementations, the core 104 is generally circular. Forexample, the back portion 120 can be disk-shaped, the center post 122can be circular, and the annular rim 124 can similarly be ring-shaped.However, other configurations are possible that maintain an annularconfiguration for the rim 124, e.g., the center post 122 could be asquare and the rim 124 could similarly trace the perimeter of a square.

The center post 122 has a width (W1) and the annular rim 124 has a width(W2), each of which can be measured along a direction parallel to thetop surface of the platen, e.g., parallel to the faces of the substrateand polishing pad during the polishing operation, and are substantiallylinear and extend in parallel to each other. The width W1 of the centerpost 122 can be substantially the minimum possible while providing thenecessary magnetic flux for a clear signal.

The annular rim 124 is separated from the center post 122 by a gaphaving a width (W3). The width of the gap 126 can be substantially theminimum possible while providing room for the winding assembly 130 tofit in the gap 126. For example, the width (W4) of the winding can be atleast 80%, e.g., about 90%, of the width of the gap 126. This maintainsthe magnetic field in a region close to the center post 122, andincreases spatial resolution. In some implementations, the outer surfaceof the winding assembly 130 contacts the inner surface of the annularrim 124.

The widths W1, W2 and W3 can be selected such that the surface area ofthe annular rim 124 is larger, e.g., at least two times larger, e.g., atleast three times larger, e.g., at least four times larger, than thesurface area of the center post 122. This permits more flux lines to becollected and pushed toward the inner diameter of the annular rim 124,thereby further improving the spatial resolution.

For center post having a larger width, e.g., where W1 is 3 mm or larger,the surface area of the annular rim 124 can be at least two times, e.g.,two to three times, larger than the surface area of the center post 122.For this case, the width W3 can be up to 1 mm. For center post having alarger width, e.g., where W1 is less than 3 mm, the surface area of theannular rim 124 can be at least four times, e.g., four to six times,larger than the surface area of the center post 122. For this case, thewidth W3 can be up to 2 mm.

The annular rim 124 can have a width W2 that is larger than the half thewidth W1 (e.g., larger than the radius) of the center post 122. In someimplementations, the annular rim 124 has a width W2 that is larger thanthe width W1 (e.g., larger than the diameter) of the center post 122,e.g., at least 10% larger. For example, the center post 122 can have awidth of 1.5 mm, the gap 126 can have a width of about 1 mm, and theannular rim 124 can have a width of about 1.75 mm.

The center post 122 and the annular rim 124 have a height Hp, which isthe distance that the they extend from the back portion 120 of the core104. The height Hp can be greater than the widths W1 and W2. In someimplementations, the height Hp is the same as the distances W3separating the prongs 504 a-c.

In general, the in-situ eddy current monitoring system 400 isconstructed with a resonant frequency of about 50 kHz to 50 MHz. Forexample, for the eddy current monitoring system 400 shown in FIG. 4A,the coil 422 can have an inductance of about 0.1 to 50 microH, e.g.,0.75 uH, and the capacitor 424 can have a capacitance of about 40 pF toabout 0.022 uF, e.g., 150 pF.

The electromagnetic induction monitoring system can be used in a varietyof polishing systems. Either the polishing pad, or the carrier head, orboth can move to provide relative motion between the polishing surfaceand the substrate. The polishing pad can be a circular (or some othershape) pad secured to the platen, a tape extending between supply andtake-up rollers, or a continuous belt. The polishing pad can be affixedon a platen, incrementally advanced over a platen between polishingoperations, or driven continuously over the platen during polishing. Thepad can be secured to the platen during polishing, or there can be afluid bearing between the platen and polishing pad during polishing. Thepolishing pad can be a standard (e.g., polyurethane with or withoutfillers) rough pad, a soft pad, or a fixed-abrasive pad.

In addition, although the description above has focused on polishing,the core design can be applicable to in-situ monitoring during othersubstrate processing tools and steps that modify the thickness of thelayer on the substrate, e.g., etching or deposition, and to in-line orstand-alone system measurements.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. An apparatus for chemical mechanical polishing,comprising: a support for a polishing pad having a polishing surface;and an electromagnetic induction monitoring system to generate amagnetic field to monitor a substrate being polished by the polishingpad, the electromagnetic induction monitoring system comprising a coreand a coil wound around a portion of the core, the core including a backportion, a center post extending from the back portion in a firstdirection normal to the polishing surface, the center post having afirst width in a second direction parallel to the polishing surface, andan annular rim extending from the back portion in parallel with thecenter post and surrounding and spaced apart from the center post by agap, the annular rim having a second width in the second direction andthe gap having a third width in the second direction, wherein the thirdwidth is less than the first width, and a surface area of a top surfaceof the annular rim is at least two times greater than a surface area ofa top surface of the center post.
 2. The apparatus of claim 1, whereinthe second width is greater than the first width.
 3. The apparatus ofclaim 2, wherein the second width is 1.1 to 1.5 times greater than thefirst width.
 4. The apparatus of claim 1, wherein the third width is 50%to 75% of the first width.
 5. The apparatus of claim 1, wherein thesurface area of the top surface of the annular rim is at least threetimes greater than the surface area of the top surface of the centerpost.
 6. The apparatus of claim 1, wherein a height of the center postis equal to a height of the annular rim.
 7. The apparatus of claim 1,wherein the third width is between about 30% and 70% of the secondwidth.
 8. The apparatus of claim 1, wherein the coil and core areconfigured to provide a resonant frequency of 50 kHz to 50 MHz.
 9. Theapparatus of claim 8, wherein the coil and core are configured toprovide a resonant frequency between about 14 and 16 MHz.
 10. Theapparatus of claim 1, wherein the core is nickel zinc ferrite ormagnesium zinc ferrite.
 11. An apparatus for chemical mechanicalpolishing, comprising: a support for a polishing pad having a polishingsurface; and an electromagnetic induction monitoring system to generatea magnetic field to monitor a substrate being polished by the polishingpad, the electromagnetic induction monitoring system comprising a coreand a winding assembly, wherein the core includes a back portion, acenter post extending from the back portion in a first direction normalto the polishing surface, the center post having a first width in asecond direction parallel to the polishing surface, and an annular rimextending from the back portion in parallel with the center post andsurrounding the center post and spaced apart from the center post by agap, the annular rim having a second width in the second direction andthe gap having a third width in the second direction, and wherein thewinding assembly is a cylindrical body fitting in the gap, the windingassembly includes a coil wound around the center post, the windingassembly has a fourth width between an inner diameter and an outerdiameter of the cylindrical body, and wherein the fourth width is atleast 80% of the third width.
 12. The apparatus of claim 11, wherein thewinding assembly includes a bobbin, the coil is wound around the bobbin,and an inner surface of the bobbin provides the inner diameter of thewinding assembly.
 13. The apparatus of claim 12, wherein the innersurface of the bobbin contacts an outer surface of the center post. 14.The apparatus of claim 11, wherein the winding assembly includes a tapecontacting and surrounding the coil, and an outer surface of the tapeprovides the outer diameter of the winding assembly.
 15. The apparatusof claim 14, wherein the outer surface of the tape contacts an innersurface of the annular rim.
 16. The apparatus of claim 11, wherein thecoil comprises no more than two winding layers around the center post.17. The apparatus of claim 14, wherein the coil comprises a singlewinding layer around the center post.
 18. The apparatus of claim 11,wherein the fourth width is at least 90% of the third width.
 19. Theapparatus of claim 11, wherein the third width is about 1 to 2 mm. 20.The apparatus of claim 11, wherein the third width is less than thefirst width, and a surface area of a top surface of the annular rim isat least two times greater than a surface area of a top surface of thecenter post.